The Hippo tumor suppressor pathway functions to limit tissue growth and organ size by inhibiting proliferation and inducing apoptosis. Dysregulation of the Hippo pathway contributes to tumorigenesis. The key downstream effectors of the Hippo pathway are the transcription co-activator YAP, which is phosphorylated and inhibited by the Hippo pathway kinase Lats. YAP overexpression and hyperactivation are found in human cancers. Extensive studies have identified many intracellular proteins that modulate the Hippo pathway. However, key questions regarding the extracellular signals and cell surface receptors for the Hippo pathway have not been addressed. We recently discovered that G-protein coupled receptors (GPCR) and their cognate ligands regulate the Hippo pathway. GPCR modulates many intracellular signaling molecules including protein kinase A (PKA) and protein kinase C (PKC). PKA is activated by cAMP, a second messenger that is elevated by stimulation of Gs-coupled receptor. PKC is activated by diacylglycerol that is also a second messenger elevated by Gq/11- coupled receptors. Both PKA and PKC are involved in a wide range of cellular regulation, including gene expression and cell growth. Our preliminary studies reveal that PKA and PKC potently modulate YAP. PKA inhibits YAP by increasing phosphorylation while PKC activates YAP by inducing dephosphorylation. The long- term goal of this project is to elucidate the mechanism of YAP regulation by PKA and PKC, to understand the regulation and function of the Hippo-YAP pathway in cell growth, organ size, tumorigenesis and cancer metastasis, and to provide potential therapeutic targets for cancer treatment.